improve parser for expressions, removing a previous limitation

[edg-l]

The previous limitation is that you could only do 4 + 4 and not 4 + 4 + 4, now its fixed, it also has correct precedence of operations (the precendece i used is like the one from C languages, but needs a proper specification).

You can now do things like

my_func((4 * 2) / 5 + 2 + 4 - (-5 - -6));